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Preheat the oven to 350°F. Generously grease every crevice of a 10- to 12-cup Bundt pan.
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In a large bowl, toss the apples with the brown sugar, cinnamon, melted butter, lemon juice, salt, and walnuts. Let sit for 10 minutes, stirring once.
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In a separate bowl, whisk together the flour, baking powder, baking soda, nutmeg, and salt.
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Beat the softened butter and granulated sugar until light and fluffy, about 3 minutes. Beat in the eggs one at a time. Mix in the vanilla and vegetable oil.
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Add the dry ingredients in three additions, alternating with the sour cream. Mix just until combined.
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Spoon about half of the apple-walnut mixture evenly into the bottom of the Bundt pan. Fold the remaining apple mixture and juices into the batter.
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Spoon the batter over the apples in the pan and smooth the top.
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Bake for 55–65 minutes, until a skewer inserted into the cake comes out clean or with a few moist crumbs.
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Cool in the pan for 10–15 minutes. Run a thin spatula around the outer edge and center tube, then invert onto a serving plate. Lift the pan away slowly.